Electromagnetic Compatibility (EMC) refers to the ability of an electronic device or system to function properly in its electromagnetic environment and does not constitute unacceptable electromagnetic harassment to anything in the environment. It includes both electromagnetic interference (EMI) and electromagnetic sensitivity (EMS). EMI refers to electrical products that cause interference. EMS refers to the ability of electrical products to resist electromagnetic interference. A device with good electromagnetic compatibility should not be affected by the surrounding electromagnetic noise nor cause electromagnetic interference to the surrounding environment.
The three elements of electromagnetic interference are interference sources, coupling channels and sensitive bodies. Suppression of the interference generated by the switching power supply is of great significance for ensuring the stable and stable operation of the electronic system. The electromagnetic interference suppression technologies mainly include weakening the interference energy, isolating and weakening the noise coupling path, and improving the resistance of the device to electromagnetic disturbance. This paper analyzes the causes of electromagnetic interference in switching power supplies, and introduces the electromagnetic interference suppression technologies and design methods for switching power supplies.
1 switching power supply electromagnetic interference
The switching power supply usually rectifies the commercial frequency alternating current into direct current, then turns it into high frequency through the control of the switch tube, and then outputs through the rectifying and filtering circuit to obtain a stable DC voltage. The industrial frequency rectification and filtering use high-capacity capacitors to charge and discharge, the high-frequency on-off of the switch tube, and the output rectifier diode reverse recovery and other work processes produce high di/dt and du/dt, forming a strong inrush current. And the peak voltage, which is the basic reason for the electromagnetic interference generated by the switching power supply. In addition, the driving waveforms of the switch tubes, the MOSFET drain-source waveforms, and the like are nearly periodic wave-shaped periodic waves. Therefore, the frequency is in the MHz range. These high-frequency signals interfere with the basic signals of the switching power supply, especially the signals of the control circuit.
1.1 Input rectifier circuit harmonic interference
Switching power supply input is usually bridge rectifier, capacitor filter circuit. The rectifier bridge can only conduct when the ripple voltage exceeds the voltage on the input filter capacitor, the current is input from the mains supply, and the filter capacitor is charged. Once the voltage on the filter capacitor is higher than the instantaneous voltage of the mains supply, the rectifier is turned off. Therefore, the current in the input circuit is pulsed and it has a rich and efficient harmonic current. This is because of the non-linear characteristics of the rectifier circuit, and the AC side current of the rectifier bridge is seriously distorted.
The number of harmonics on the DC side is n times. Therefore, the high-frequency harmonic current on the DC side of the rectifier circuit not only generates power in the circuit, but also increases the reactive power of the circuit, and high-frequency harmonics can generate conducted and radiated interference along the transmission line.
1.2 Interference from Switching Circuits
Switching circuits play a key role in switching power supplies and are also one of the main sources of interference. The switch load is the primary coil of the high-frequency transformer, which is an inductive load. At the moment of conduction, the primary coil generates a large inrush current, and a high surge voltage appears across the primary coil. At the moment of disconnection, due to the leakage flux of the primary coil, part of the energy is not transmitted from the primary coil. Transferred to the secondary coil, this part of the energy stored in the inductor will oscillate with the attenuation of the capacitor and resistor in the collector circuit to form a sharp peak, superimposed on the turn-off voltage to form the turn-off voltage spike. If the spike has a high enough amplitude, it is very likely to break the switch.
1.3 Common Mode Conducted Harassment Generated by High Frequency Transformers
High-frequency transformer is an important part of the switching power supply for energy storage, isolation, output, and voltage conversion. Its leakage inductance and distributed capacitance have a great influence on the electromagnetic compatibility of the circuit. Because the primary coil has leakage magnetic flux, part of the energy is not transmitted to the secondary coil, but is formed by the capacitor and resistor in the collector circuit to form a damped oscillation with a peak, superimposed on the turn-off voltage to form a turn-off voltage spike. The same magnetizing impulse current transient occurs when the primary coil is turned on. This noise will be conducted to the input and output terminals to form conduction disturbances, and it is likely to cause breakdown of the switch tube. In addition, the high-frequency switching current loop formed by the primary coil, switching tube and filter capacitor of the high-frequency transformer may generate large space radiation and cause radiation disturbance.
There is a distributed capacitance between the primary and secondary of the frequency modulation transformer of the switching power supply. Using a device capacitor (device-to-ground distributed capacitance) to be equivalent to the entire switching power supply, an interference channel is formed. The common-mode interference passes through the coupling capacitor of the transformer, returns to earth through the device capacitance, and a voltage divider formed by the coupling capacitor of the transformer and the capacitor of the device is obtained. The high-frequency switching current loop formed by the primary coil of the pulse transformer, the switch tube and the filter capacitor may generate large space radiation and form radiation disturbance.
1.4 Distribution and parasitic parameters caused by switching power supply noise
The distribution parameters of the switching power supply are the internal factors of most interferences. The distributed capacitance between the switching power supply and the heat sink, the distributed capacitance between the primary and secondary of the transformer, and the leakage inductance of the primary and secondary sides are all noise sources. Common-mode interference is transmitted through the distributed capacitance between the primary and secondary transformers and the distributed capacitance between the switching power supply and the heat sink. The distributed capacitance of the transformer winding is related to the winding structure and manufacturing process of the high frequency transformer. The distributed capacitance between the switching power supply and the heat sink is related to the structure of the switch tube and the installation manner of the switch tube. The use of a shielded insulating spacer can reduce the distributed capacitance between the switch tube and the heat sink.
Components that operate at high frequencies have high-frequency parasitic characteristics that affect their operating conditions. When the high-frequency operation is performed, the wire becomes an emission line, the capacitor becomes an inductance, the inductance becomes a capacitance, and the resistance becomes a resonance circuit. When the frequency is too high, the frequency characteristics of each element cause a considerable change. In order to ensure the stability of the switching power supply during high-frequency operation, when designing the switching power supply, full consideration should be given to the characteristics of the device during high-frequency operation, and a component with better high-frequency characteristics should be selected. In addition, at high frequencies, the inductive reactance of the parasitic inductance of the wire significantly increases. As a result of the uncontrollability of the inductor, it eventually becomes an emission line, which becomes a source of radiated interference in the switching power supply.
2 Measures to suppress electromagnetic interference
There are two types of electromagnetic interference in the switching power supply, common mode interference and differential mode interference. According to the electromagnetic interference sources analyzed in the foregoing, combined with their coupling paths, interference can be suppressed from several aspects such as EMI filters, absorption circuits, grounding, and shielding, and the electromagnetic interference can be attenuated to the allowable limit.
2.1 AC Input EMI Filter
Filtering is a method of suppressing conducted interference. Connecting a filter at the input of the power supply can suppress the noise from the power grid to the power supply itself, and can also suppress the interference generated by the switching power supply and fed back to the power grid. The power filter acts as an important unit to suppress the conducted interference of the power line and plays an extremely important role in the electromagnetic compatibility design of the device or system. The EMI filter circuit shown in Figure 1 is usually used at the input end of the power supply. This circuit can effectively suppress low frequency differential mode disturbance and high frequency common mode disturbance at the input end of AC power supply. In the circuit, the differential mode capacitors Cx1 and Cx2 (also called X capacitors) connected across the power supply are used to filter out differential mode interference signals. A ceramic capacitor or a polyester film capacitor is generally used. The capacitance value is usually 0.1 to 0.47F. .
The common-mode capacitors Cy1 and Cy2 (also called Y-capacitors) grounded in the middle of the connection are used to short-circuit the common-mode noise current. The value range is usually C1=C2 # 2 200 pF. Suppression inductance L1, L2 usually take 100~130H, the common mode choke coil L is made up of two coils that are identical and wound in the same direction on a magnetic core, usually require its inductance L#15~ 25 mH. When the load current passes through the common mode choke coil, the magnetic field lines generated by the coils connected in series on the hot line and the lines of magnetic force generated by the coils connected in series on the neutral line are in opposite directions, and they cancel each other out in the magnetic core. Therefore, the core does not saturate even at large load currents. For common-mode interference currents, the magnetic fields generated by the two coils are in the same direction, and they will exhibit larger inductances, which will act to attenuate common-mode interference signals.
2.2 Using absorption circuit
The main reason for the EMI generated by the switching power supply is a sharp change in voltage and current. It is therefore necessary to reduce the rate of change of the voltage and current in the circuit (du/dt and di/dt) as much as possible. The absorption circuit can suppress EMI. The basic principle is to bypass the switch when it is off, absorb the energy accumulated in the parasitic distribution parameters, and thus suppress the occurrence of interference. The RC snubber circuit shown in Figure 2(a) can be connected in parallel across the switch tube. During the turn-on and turn-off process of the switch tube or diode, the reverse peak current and peak voltage generated in the tube can be buffered. get over. The buffer absorption circuit can reduce the amplitude of the spike voltage and reduce the rate of change of the voltage waveform, which is very beneficial for the safety of the use of the semiconductor device.
At the same time, the snubber circuit also reduces the spectral content of the RF radiation, which is beneficial for reducing the energy of the RF radiation. Clamping circuits are primarily used to prevent the risk of breakdown of semiconductor devices and capacitors. 5. Taking into account the protection of the clamping circuit and the efficiency requirements of the switching power supply, the breakdown voltage of the TVS tube is chosen to be 1.5 times the induced voltage of the primary winding. When the voltage on the TVS exceeds a certain level, the device rapidly conducts, thereby discharging the surge energy and limiting the amplitude of the surge voltage to a certain extent. The saturable magnetic core coil or microcrystalline magnetic beads may be serially connected to the drain of the switch tube and the positive electrode lead of the output diode. The material is generally cobalt. When the magnetic core is saturated by normal current, the inductance is very small. Once the current has to flow in the reverse direction, it will produce a very large back EMF, which effectively suppresses the reverse surge current of the diode.
2.3 Shielding measures
The effective method of suppressing radiation noise is shielding. The electric field can be shielded with a material with good conductivity, and the magnetic field can be shielded with a material with high permeability. In order to prevent the magnetic field leakage of the transformer, the primary and secondary coupling of the transformer is good, and the closed magnetic ring can be used to form a magnetic shield. For example, the leakage magnetic flux of the can core is obviously much smaller than that of the E type. The switch power supply's connection line, power line should use the wire with shielding layer, try hard to prevent the outside disturbance from coupling into the circuit. Or use magnetic beads, magnetic rings and other EMC components to filter out high-frequency interference from the power supply and signal lines.
However, it should be noted that the signal frequency cannot be disturbed by EMC components, ie the signal frequency is within the passband of the filter. The entire switch power supply housing also needs to have good shielding characteristics. The joints must comply with the EMC requirements for shielding. Through the above measures, it is ensured that the switching power supply is neither disturbed by the external electromagnetic environment nor interferes with external electronic equipment.
2.4 Transformer Winding
Leakage inductance must be minimized when designing high-frequency transformers. Because the greater the leakage inductance, the higher the peak voltage amplitude generated, the greater the drain clamp losses, which inevitably leads to a reduction in power supply efficiency. Reducing the leakage inductance of a transformer usually involves reducing the number of turns of the primary winding, increasing the width of the winding, and reducing the insulation layer between the windings.
The main parasitic parameters of the transformer are leakage inductance, inter-winding capacitance, and cross-coupling capacitance. The cross-coupling capacitance between the transformer windings provides a path for common-mode noise to flow through the entire system.
Faraday shields are used during transformer winding to reduce cross-coupling capacitance. The Faraday shield is simply a copper foil or aluminum foil wrapped between the primary winding and the secondary winding to form a surface shield isolation zone and grounded, where the primary and secondary windings are interleaved to reduce crossover. Coupling capacitors. The heat sink grounding is generally required in the installation procedure. The parasitic capacitance between the drain and the heat sink of the switch provides a path for common mode noise. A copper foil or aluminum foil can be added between the drain and the heat sink and grounded. Reduce this parasitic capacitance.
2.5 Application of Grounding Technology
The switching power supply needs to pay attention to the connection of the ground wire. The ground wire bears the heavy responsibility of the reference level, especially the reference ground of the control circuit, such as the ground level of the current detection resistance and the ground level of the voltage divider resistance without isolated output.
(1) The signal ground of the device. The signal ground of the device may be a ground reference point of a signal or a piece of metal in the device, which provides a common reference potential for all signals in the device. Such as floating ground and mixed ground, there are also single-point grounding and multi-point grounding.
(2) The equipment is connected to earth. In engineering practice, in addition to careful consideration of the signal grounding inside the equipment, the signal ground of the equipment and the chassis are usually connected with the earth, and earth is used as the ground reference point of the equipment.
The ground level attenuation of the control signal should be as small as possible. Therefore, the control part is grounded at one point, and then the common connection point is connected to the power ground again. This grounding method can separate the noise source from the sensitive circuit. In addition, the ground lines should be as wide as possible, and the blank areas can be filled with copper to reduce ground level errors and EMI.
In the device, surface mount components are used as much as possible to make the assembly density higher, the volume smaller, the weight lighter, the reliability higher, the high frequency characteristics better, and the electromagnetic and radio frequency interference reduced.
2.6 PCB Component Layout and Traces
One of the most difficult problems to overcome in printed circuit board circuits is the strip lines, wires, and cables in the PCB. The radiation disturbance of the switching power supply is proportional to the product of the current in the current path, the loop area of ​​the path, and the square of the current frequency, so the layout design of the PCB will directly relate to the electromagnetic compatibility performance of the whole machine. When designing switching power supply printed circuit boards, it is necessary to start from the optimization design of layout and wiring.
(1) PCB layout usually meets the following principles
1. The conductors used in the input and output terminals should avoid adjacent equality as much as possible. It is best to add ground between lines to avoid feedback coupling;
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